When exposed to an air stream containing submicron particles, air filtration webs typically experience a loss of filtration efficiency. Filtration efficiency may be evaluated using a percent penetration test that uses a challenge aerosol that contains, for example, particles of sodium chloride or dioctyl phthalate. Both initial penetration and maximum penetration may be determined in accordance with such a test. Maximum penetration values are of particular interest because they present an indication of filter service life.
A variety of charging techniques have been employed to improve filtration efficiency. Certain substances such as oily aerosols, however, are known to cause a decline in electric charge over time. High initial filtration efficiencies may be achieved using charged filter media such as needled felt, or charged spunbond or meltblown webs. An undesirably high basis weight, however, may also be required, especially for charged media with larger diameter fibers. Coarse fiber charged filter media often has high initial efficiency but may experience a severe drop in efficiency as the filter accumulates very fine particles. This efficiency loss in charged filter media may be referred to as electret degradation. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) Standard No. 52.2 entitled “Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size” defines a minimum efficiency reported value (MERV) rating for heating, ventilating and air conditioning (HVAC) filters. Proposed changes in Standard No. 52.2 to account for electret degradation may change the standard so that electret filters are challenged with a greater proportion of small particles. If enacted, these changes to the standard may reduce MERV ratings for typical electret filter media by 2-3 rating points.
In general, fine fibers (e.g., nanofibers) also promote high filtration efficiency, but pressure drop typically increases as fiber diameter decreases. For example, high initial filtration efficiencies may be achieved using fiberglass composites containing submicron fibers, but these good initial filtration efficiencies are often achieved at the expense of a higher initial pressure drop. Glass fibers also are problematic in that the fibers are generally not recyclable and are prone to fracture due to their brittleness. Glass fiber fragments may also cause respiratory or epidermal irritation. Filter media made from polymeric nanofibers have been used instead of glass fibers. Polymeric nanofibers, however, have poorer chemical and solvent resistance than glass fibers. For example, polymeric nanofibers produced using electrospinning are at a minimum susceptible to the solvents from which they were spun. Also, many currently available nanofibers are typically produced at such low rates as to be excessive in cost for many applications. Electrospun nanofibers are typically produced at grams per day rates, and blown glass nanofibers are relatively expensive when compared to standard filter media. Even islands-in-the-sea nanofibers, which can be produced at high rates, are costly to produce because they require a removable sea and a process step to remove the sea.
Initial penetration and maximum penetration values may sometimes be poorly correlated. This lack of correlation makes it difficult to predict maximum penetration values based on initial penetration measurements. Maximum penetration may instead be measured, but this measurement may take a long time for media exposed to very small (e.g., submicron) particles. Filter design also may be made more difficult when a web exhibits poorly correlated initial and maximum penetration values.
Fibrous air filtration webs are described, for example, in U.S. Pat. Nos. 4,011,067 (Carey), 4,215,682 (Kubik et al.), 4,592,815 (Nakao), 4,729,371 (Krueger et al.), 4,798,850 (Brown), 5,401,466 (Tsai et al.), 5,496,507 (Angadjivand et al. '507), 6,119,691 (Angadjivand et al. '691), 6,183,670 B1 (Torobin et al. '670), 6,315,806 B1 (Torobin et al. '806), 6,397,458 B1 (Jones et al. '458), 6,554,881 B1 (Healey), 6,562,112 B2 (Jones et al. '112), 6,627,563 B1 (Huberty), 6,673,136 B2 (Gillingham et al.), 6,716,274 B2 (Gogins et al.), 6,743,273 B2 (Chung et al.) and 6,827,764 B2 (Springett et al.), and in Tsai et al., Electrospinning Theory and Techniques, 14th Annual International TANDEC Nonwovens Conference, Nov. 9-11, 2004. Other fibrous webs are described, for example, in U.S. Pat. Nos. 4,536,361 (Torobin) and 5,993,943 (Bodaghi et al.).